1. Field of the Invention
The present invention relates to an automatic gain control (AGC) circuit of a receiver that includes a mixer for selectively using two or more local oscillation frequencies to perform frequency conversion.
2. Description of the Related Art
FIG. 1 shows a circuit configuration of a portion of the front end of a conventional receiver, that includes a mixer circuit and an AGC circuit.
In FIG. 1, a reception signal with frequency fr received at an antenna 1 is supplied to a mixer 3 via a gain control circuit 2. Meanwhile, a local oscillation frequency signal with frequency fl is supplied to the mixer 3 from a local oscillation circuit 4.
The mixer 3, which comprises a double balanced modulator or a ring modulator that uses diodes or transistors, for example, generates an intermediate frequency signal having frequency components of the sum and difference of the frequencies of both signals, namely |fr±fl|. In other words, the reception signal frequency is converted to an intermediate frequency suitable for the subsequent demodulation/reproduction processing by passing through the mixer 3.
The intermediate frequency signal that is output from the mixer 3 is supplied to a demodulation/reproduction circuit 7 in the subsequent stage and also to a detector 5. The detector 5 detects the supplied intermediate frequency signal, detects its signal level, and outputs the relevant signal level to an AGC control circuit 6. The AGC control circuit 6 controls the start of the execution of AGC operation in the gain control circuit 2 according to the relevant signal level.
By performing this kind of feedback control, the output signal level from the mixer 3 can be maintained at a prescribed value even if the signal level of the reception signal fluctuates due to variations in the reception electric field intensity.
Incidentally, the radio waves from the antenna 1 include various interference waves, not just the desired waves that are to be received. For example, if a signal that is modulated according to the orthogonal frequency division multiplexing (OFDM) method (hereafter, referred to simply as an “OFDM modulation signal”) is received as an interference wave as shown in FIG. 2A, the following kind of problem occurs. More specifically, the OFDM modulation method uses a so-called multi-carrier method in which a plurality of carriers with several different frequencies are arrayed in the band. Consequently, if the signal level of the OFDM modulation signal that is the interference wave grows to a certain degree, an intermodulation distortion that occurs between the carriers will increase as shown in FIG. 2B. As a result, its frequency spectrum may spread to the frequency band of a desired wave, which may hinder the reception of the desired wave. Therefore, the concept of a so-called broadband AGC has been introduced in the AGC control scheme described earlier, and a method has been used that attends not just to the signal level of the desired wave but also to the signal level of the interference wave in order to attenuate the input signal level of the interference wave by using the gain control circuit 2.
However, in a receiver that has two local oscillation frequencies, for example, and that switches between these frequencies, the following kind of problem will occur. The intermediate frequency that is generated by the mixer 3 when the reception signal is frequency converted by using the local oscillation frequency fl and the intermediate frequency that is generated when it is frequency converted by using the local oscillation frequency fu will naturally differ.
Now, assume that the intermediate frequency of the frequency spectrum of an OFDM modulation signal that is the interference wave from antenna 1 is fi and that the input level is χ dBm. Also, let G(f) dB be the detector 5 input from antenna 1 in frequency f, that is, the conversion gain up to the mixer 3 output. In this case, the input signal level Ll to the detector 5 of the interference wave that was frequency converted by using local oscillation frequency fl will be as follows.Ll=G(|fi−fl|)+X dBm
On the other hand, the input signal level Lu to the detector 5 of the interference wave that is frequency converted by using local oscillation frequency fu will be as follows.Lu=G(|fi−fu|)+X dBm
Since the conversion gain G(f) up to the mixer 3 output from the antenna 1 generally has a frequency characteristic, the values of G(|fi−fl|) and G(|fi−fu|) in the above equations will differ. Therefore, the following inequality holds:Ll≠Lu
and the signal level of the interference wave that is input to the detector 5 when the interference wave fi is frequency converted by using fl will differ from the signal level when fi is frequency converted by using fu.
Therefore, if a setting is required for starting AGC operation according to gain control circuit 2 when the antenna input of the interference wave exceeds χ dBm, for example, then if the relevant setting is made by using the input signal level Ll to the detector 5 when frequency conversion is performed by using fl, for example, the AGC will not operate even if the antenna input of the interference wave exceeds χ dBm when frequency conversion is performed by using fu.
As a result, a problem occurs that, when the user changes a reception channel for which local oscillation frequency switching is required, the broadcast that could be received up to then because AGC operated will not be able to be received.